Of a Rhombohedral (Pseudocubic) Unit of Structure with a = 5.08 a and a = 90

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TABLE 2 TUMPER- LOG K02 LOG KO0 LOG KON ATURS L AND R, B AND R, B AND R, ABS. p. 485 p. 96 LOG KOHB p. 96 LOG KOHV LOG KiB LOG KV LOG K4B LOG K4V 1000 -19.8 -20.1 -21.8 -21.1 -15.5 +1.95 -4.35 +2.74 -3.55 1300 -13.9 -14.01 -15.2 -14.3 -10.34 +1.31 -3.53 +0.30 -4.54 1500 -11.2 -11.42 -12.24 -11.4 - 8.05 +1.03 -3.16 -0.79 -4.98 1705 - 9.13 - 9.28 - 9.95 - 9.0 - 6.26 +0.81 -2.87 -1.66 -5.35 1900 - 7.55 - 7.6 - 8.21 - 7.2 - 4.90 +0.65 -2.66 -2.29 -5.60 interp. 2155 - 5.94 - 6.08 - 6.42 - 5.5 - 3.50 +0.48 -2.43 -2.99 -5.91 2505 - 4.27 - 4.31 - 4.59 - 3.5 - 2.08 +0.32 -2.19 -3.67 -6.18 20H > H2 + 02 + 14,000 cal. (B) 20H - 11 + 02 - 15,000 cal. (V) 1 Villars, J. Am. Chem. Soc., 51, 2374-7 (1929). 2 Dr. Langmuir has kindly pointed out that the knowledge of such equilibrium constants will be extremely useful in investigating the possibility of the direct formation of hydroxyl during the combustion of hydrogen and oxygen at high temperatures, an investigation which he is at present carrying on. Z. physik. Chem., 139A, 75-97 (1928). 4 Z. Physik, 49, 465. 5 Cf. Rodebush's chapter XVII, p. 1202, in Taylor, A Treatise on Physical Chemistry, D. Van Nostrand, 1925. 6 Lewis and Gibson, J. Am. Chem. Soc., 39, 2554 (1917). 7 Mulliken, Phys. Rev., 32, 186. 8 J. Am. Chem. Soc., 50, 3221. s These PROcEgDINGS, 15, 678-680 (1929). 10 Rodebush, loc. cit., obtained this value and showed that the experimental method used to determine the entropy of hydrogen is theoretically unsound. "Prof. Rodebush has shown independently that this relation must obtain.

ON THE CRYSTAL STRUCTURE OF THE CHLORIDES OF CERTAIN BIVALENT ELEMENTS By LINUS PAULING GATES CHEMIcAL LABORATORY, CALIFORNIA INSTITUTE OF TECHNOLOGY Communicated August 6, 1929 In 1925 Bruni and Ferrari' reported that the lines on a powder photograph of the hexagonal crystal MgCl2 could be accounted for on the basis of a rhombohedral (pseudocubic) unit of structure with a = 5.08 A and a = 90. containing 2 MgCl2. Later they announced2 that ZnCl2, CdCI2, and MnCl2 have the same structure, with values of a a few degrees larger than 90 . Ferrari3 then added CoC12 and NiCl2 to the group; certain weak lines occurring on the powder photographs of these substances re- quire the value of a to be doubled, giving a rhombohedral unit with a = 900 Downloaded by guest on September 29, 2021 710 CHEMISTRY: L. PA ULING PRoc. N. A. S. and a = 10.02 A and 10.00 A, respectively, containing 16 M++Cl2. Gold- schmidt4 has also reported that unpublished powder photographic data obtained by Oftedal in his laboratory show that RuC12, RhC12, PdC12, IrCl2, and PtCl2 are to be assigned the same structure as NiC12. None of these investigators succeeded in deducing the atomic arrange- ment in these crystals; the utilization of Bruni and Ferrari's data to this end is described in the following paragraphs. Bruni and Ferrari published reproductions2 of Laue photographs of CdCl2, MgC12 and MnCl2, taken in order to determine the axial ratios for these crystals. I have assigned indices to the spots appearing on the best of these photographs, that of CdCl2, with the aid of a gnomonic projection made from a tracing. The forms producing these spots and values of nX calculated for a unit with a = 940 and a = 10.2 A (an estimated value, about equal to that for ZnCl2) are given in the first and second columns of table 1. Only a few forms show nX values below 0.4 A, indicating first-order reflections. It is, indeed, possible to account for the production of all of the spots on the photograph on the basis of a much smaller unit, a rhombohedron with a = 6.35 A and a = 360 40', containing 1MCl2. This unit has lattice points at the face-centered positions of the rhombohedron which is itself formed by the face-centered positions of the original pseudocubic unit. Inasmuch as values of nX calculated for this unit (fourth column of table 1) in no case fall below the short wave-length limit, the unit may be accepted as the true one. The presence of a three-fold axis and three planes of symmetry in the Laue photograph taken with the incident beam normal to the basal plane requires that the crystal be isomorphous with one of the point-groups C3,, D3 and Dv. The fundamental lattice has been found to be rhombohedral. The only ways of placing 1CdCl2 in the unit compatible with these restrictions are I Cd at 000, 2 C1 at uuu, fifwi; D' and D5d; II Cd at 21, 2 C1 at uuu, unuu; D7 and D'd; III Cd at uuu, C1I at vvv, CII, at www; C3v, In the absence of any evidence requiring it, the three-parameter structure III need not be considered. Structures I and II are identical. The data are not sufficient for an accurate determination of the value of the chlorine parameter u. The fact that the powder photograph of MgCl2 is nearly identical with that of LiCl indicates that the chlorine atoms are in nearly the same positions in the two crystals, namely, at the points of a face-centered cubic lattice. This corresponds to a value of u of 1/4.- Further evidence that the chloride Downloaded by guest on September 29, 2021 VOL. 15, 1929 CHEMISTRY: L. PA ULING 711 ions are in approximate cubic close-packing is provided by the considera- tion of interionic distances. With a crystal radius of 1.81 A,' 32 chloride ions would occupy a cube 10.24 A on an edge; this approximates closely the values reported for the various bivalent chlorides. TABLE 1 DATA FROM BRUNI AND FERRARI'S LAUB PHOTOGRAPH OF CADMIUM CHLORIDE PSOUDOCUBIC UNIT TRUE UNIT RSTIMATED STRUCTURE a = 10.2 A a = 940 a = 6.36 A, a = 36°40' INTENSITY* FACTOR (hklI nf{hkl}0) 335 0.33 A 111 0.33 A m. Cd-2C1 331 0.68 211 0.34 w. Cd - 2C 557 0.40 221 0.40 m. Cd 122 1.44 321 0.36 M.s. Cd + 2C1 779 0.32 331 0.32 v.w. Cd 320 1.04 411 0.26 v.w. Cd-2C1 731 0.72 631 0.36 v.w. Cd - 2C1 411 1.52 611 0.38 m. Cd+2C1 432 1.40 750 0.35 m.w. Cd + 2CI 611 1.40 10.3.3 0.35 v.v.w. Cd +2C1 111 4.44 110 2.22 m. 311 1.38 100 1.38 s. 310 2.64 521 0.66 m. 321 2.02 530 0.50 m.w. 511 1.52 411 0.76 v.v.w. 531 1.20 210 1.20 w. 753 0.80 320 0.80 m.w. * V.s. = very strong, m = medium, w = weak, etc. Structure factors calculated for structure I with u = 0.25 are included in table 1 for forms giving first-order reflections. (These forms are ar- ranged in order of decreasing interplanar distance.) These are in satis- factory agreement with intensities estimated from the reproduction of the photograph. Similar agreement is obtained with the powder photograph intensities for MgCl2, ZnCl2, CoCl2 and NiC12 reported by Bruni and Ferrari.6 The structure found for this series of divalent chlorides is shown in the figure. It is a layer structure closely related to that of cadmium iodide. Each cation is surrounded by six chloride ions approximately at the corners of a regular octahedron, six edges of which are shared with other octahedra in such a way as to form a layer. In the cadmium iodide structure these layers are superimposed in such a way that each cation is directly, above a cation in the layer below, whereas in the cadmium chloride structure each cation is directly above an anion in the layer below, and below an anion in the layer above. It is probable that the cadmium chloride structure is stable for substances MX2 in which the cation M has a coordination number 6 and the anion has an appreciable but not great polarizability. Downloaded by guest on September 29, 2021 712 CHEMISTRY: L. PA ULING PROC. N. A. S. I propose to carry on the experimental investigation of some of these crystals in order to obtain accurate values for the parameter. The dis- cussion of interionic distances can be postponed until the results of this study are reported in the Zeitschrift fur Kristaliographie.

FIGURE I The structure of cadmium chloride and other bivalent chlorides. Large open circles represent chloride ions, small closed circles cadmium ions. Heavy lines mark the rhombohedral unit of structure, and light lines the coordinated octahedron formed by six chloride ions about each cadmium ion. Summary.-With the aid of data obtained from a reproduction of a Laue photograph published by Bruni and Ferrari, it is shown that the unit of structure of CdCl2 is a rhombohedron with a = 360 40' and a = 6.35 A, about, containing 1CdCl2, Cd at 000, 2C1 at uuu, uftfif, with U = 1/4. The isomorphous crystals MgCl2, CoCl2, and NiCl2 have a = 330 36' and a = 6.21, 6.14 and 6.13 A, respectively. Other representa- tives of the structure are MnCl2, ZnCl2, RuCl2, RhCl2, PdCl2, IrCl2 and PtCl2. 1 G. Bruni and A. Ferrari, Rendiconti Acc. dei Lincei, 2, 457 (1925). 2 G. Bruni and A. Ferrari, Ibid., 4, 10 (1926). 3A. Ferrari, Ibid., 6, 56 (1927). 4 V. M. Goldschmidt, Geochemische Verteilungsgesetze der Elemente, VIII, p. 147. 5 L. Pauling, J. Am. Chem. Soc., 49, 765 (1927). 6 One line of small intensity, assigned indices 14221 by Ferrari, is not compatible with the unit containing 1MC12. Downloaded by guest on September 29, 2021